Light emitting diodes (LEDs) are replacing conventional light sources such as fluorescent and incandescent light bulbs in many applications. LEDs have similar electrical efficiency and longer lifetimes than fluorescent light sources. In addition, the driving voltages needed are compatible with the battery power available on many portable devices.
To provide a replacement light source, however, light sources that utilize multiple LEDs are typically required. LEDs emit light in relatively narrow wavelength bands. Hence, to provide a light source of an arbitrary color, arrays of LEDs having different colors are often utilized.
In addition, to provide an LED light source of the intensity available from a conventional light source, multiple LEDs of each color must be included. The maximum light intensity from an LED is typically less than that available from an incandescent light of a few watts. Hence, to provide the equivalent of a 100-watt light bulb, a large number of low power LEDs must be combined in the replacement light source.
To provide a commercial light source, the LED dies must be packaged to protect the dies from the environment and provide structures that increase the amount of light that leaves the LEDs in the desired direction. For example, the narrow emission band limitation of LEDs can be overcome by including a layer of phosphor that converts a portion of the light from the LED to light having a broader spectrum. For example, a blue emitting LED covered by a phosphor that converts blue light to yellow light is perceived as white by a human observer if the phosphor layer is chosen to provide the correct ratio of blue light and yellow light in the light leaving the phosphor layer.
In addition, a significant fraction of the light generated in the LED remains trapped within the LED and is either absorbed or emitted through the side surfaces of the die. Hence, packages that provide reflectors that redirect the light leaving the side surfaces to the forward direction are often utilized. The amount of light that is trapped in the LED can also be reduced by providing a layer of transparent material over the LED in which the outer surface of the layer is approximately spherical in shape.
Heat dissipation is also an important factor in the design of LED light sources that are to replace conventional incandescent light sources. While individual LEDs dissipate only a few watts of power at best, the dies are quite small, and hence, the heat generated is localized in a very small area. If the heat is not efficiently removed, the temperature of the LEDs will increase. Such temperature increases lead to lower conversion efficiencies and lifetimes. Hence, designs that provide some form of heat-conducting path under the LED are preferred.
The above-described packaging considerations all add to the cost of the final light source. Since cost is critical in providing a replacement to low-cost conventional light sources, inexpensive packaging systems that can provide the functions described above are needed.